Since ancient times natural stones, particularly granite, marble, and limestone, have been preferred materials for cladding exterior walls of buildings. Today there are various conventional methods of cladding exterior building walls with natural stone. The conventional cladding usually employs panels of stone 1¼″ (16 psf) to 2″ (26 psf) and sometimes 3″ (39 psf) and 4″ (52 psf) thick whose weight (herein termed the dead loading, a term commonly used in the industry) must be carried by relieving angles or shelf angles which are attached to the building structure by mechanical means. The aforementioned weights (pounds per square foot or “psf”) are approximate and vary with the type of stone.
The resistance to lateral loading (herein termed the live loading) is usually accomplished by stainless steel clips, dowels or anchors inserted into kerfs or holes drilled or cut into the edges of the stone panels and connected to the building structure by mechanical means thus providing the essential mechanical connection between the stone and the structure. A structural weak point in the conventional stone construction occurs at these kerfs or anchor holes in the edges of the stone slabs and they must leave enough stone thickness to provide sufficient strength within the remaining stone thickness to resist the various wind, seismic and atmospheric pressures (live loads), both positive and negative, which will be exerted on the stone panels by forces of nature as well as stresses applied during construction handling.
Calculation of this strength is an inexact engineering task since the stone is a product of nature and properties vary from stone to stone and piece to piece. Different types of stone have different physical and structural characteristics. Weak points or hidden fractures are sometimes difficult to visually ascertain in a material such as natural stone. Mechanical values and properties of the stones used for structural or engineering calculations are obtained by means of empirical testing in laboratories and field testing on samples of particular stones and the resulting values used for structural calculations usually include a substantial safety factor in order to compensate for the unpredictability of designing with natural stone. In the design of conventional stone work, these calculations determine the thickness of stone to be used or the frequency of anchors in the edges of the stone panels. As the load factors go up the stone thickness is usually increased to add strength.
The fixing method described above for individual stone panels is often used in a pre-assembly of multiple stone panels of thickness of 1¼″ thick or greater affixed to a prefabricated steel frame or truss made up of structural steel angles, channels, beams, or steel studs to form a structural unit perhaps one or two stories high and various widths usually from column to column. This system is generally referred to as a “truss” or “strong-back” system. These preassembled, or prefabricated, panels can sometimes include windows. This method offers economies of factory assembly and rapid erection time at the jobsite. In another method sometimes used in high rise curtainwall cladding the panels of stone can be incorporated into the aluminum window framing usually by means of inserting a flange of the aluminum frame into a continuous slot which has been cut into the edge of the stone panel. This is usually referred to as a “glazed-in” system. The stone thickness for this method is usually 1¼″ or greater and the aluminum window frame must be structurally designed to carry the substantial weight of the stone panel. A disadvantage of this traditional method of fixing is the vulnerability of the stones to breakage which can occur during construction handling or from various forces such as structural movements caused by earthquake or other factors. Also it could be somewhat difficult to replace a stone panel in the event of damage or breakage without replacing the complete window frame.
Once the stone panels are set in place on the building wall by various methods as discussed above, the joints between adjacent stone panels and between stone and window frames are usually sealed or caulked with an elastomeric sealant in order to form a weather tight exterior wall surface. This is generally referred to as the “wet seal” method and in order to assure the critical watertight integrity of the facade it is necessary to provide a suitable pocket between panels for the application of the caulking sealant. This caulking process requires a depth of about 1″ in the joint to allow the placement of a compressible polystyrene backer rod to the correct depth in the joint cavity in order to provide a stopper for the sealant. The conventional systems using stones 1¼″ and 2″ thick provide adequate joint depth for this caulking method.
There have been other methods of attaching the thicker traditional stone to a prefabricated structural frame as described in U.S. Pat. Nos. 5,239,798 and 5,379,561 both issued to Saito in 1993 and 1995 respectively wherein threaded studs or bolts are fixed into undercut holes on the backside of the stone panel but this method has not been widely used as there are many disadvantages to this system.
Another prior art method to use a thinner stone veneer on a prefabricated panel is described in U.S. Pat. No. 4,506,482 issued to Hans J. Pracht et al in 1985. In this method the structure consisted usually of a steel stud frame wall with an attached metal decking platform to receive the facing veneers which were generally tiles of various materials and dimensions and which were resiliently bonded to the steel decking with a structural silicone. The silicone adhesive was the sole support and attachment of the facing veneer for both the dead loads and the live loads. In the case of natural stone, it was necessary to reduce the dead weight as much as possible. Therefore the stone veneers often consisted of tiles of small thickness such as ⅜″ or ½″ and small dimensions such as 12″×12″ or 16″×16″. To use larger dimension panels it was necessary to use thicker slabs such as ¾″, 1″, or 1¼″ usually with a shelf angle to carry the extra weight. The U.S. Pat. No. 4,783,941 issued to William Loper et al in 1988 and commercialized as the “Cygnus Panel System” was considered an improvement over the previously mentioned U.S. Pat. No. 4,506,482 and essentially added metal clip attachments usually in kerfs in the edges of the stone panels which were then connected to the steel decking on the panel structure. This provided a positive mechanical connection to the structure in order to carry the extra weight which was useful in situations where building codes require mechanical connections between stone veneer and building structure. Both of these methods are comprised of a prefabricated structural panel with a plurality of veneer panels. As such, there are inherent limitations in the flexibility or adaptability of this type of panel to resolve many of the design conditions found in today's building facades. While this type of panel can be useful for new construction, and particularly for mid to high-rise buildings, it has a very limited use in renovation work. A major contribution of these methods lies in the advancement of the use of structural silicone adhesive as a means of resilient attachment of stone in building facades. The silicone adhesive has been in accepted use for more than 40 years to attach large panes of window glass on high-rise building curtain walls. But primarily because of the excessive weight of conventional stone panels this adhesive was not heretofore widely used to support stone on building facades.
Another prior art method of exterior cladding with stone involves lightweight panels made up of a very thin veneer of stone which is adhered with epoxy to a sandwich panel of aluminum honeycomb between two layers of fiberglass. A method of fabricating these panels is discussed in U.S. Pat. Nos. 5,243,960 and 5,339,795 issued to Peter Myles in 1993 and 1994 respectively and they are presently commercialized by Stone Panels Inc. These panels are about 1″ thick and are usually installed on a building facade by means of a modified aluminum C-shaped clip or interlocking channel attached to the back of the stone faced honeycomb panel with an epoxy set threaded insert. This channel interlocks with matching aluminum runners which are installed on the building and the panels are hung on the runners. One potential problem with this system is the fact that the very thin veneer of stone, only about 3/16″ thick, is adhered to the honeycomb panel only by the epoxy adhesive and could possibly delaminate over time due to constant exposure to the elements or the differential expansion between stone and the fiberglass covered honeycomb panel due to thermal extremes. A second potential problem is the inability to provide a positive mechanical connection between the very thin stone veneer, only 3/16″ thick, and the building structure which would keep the stone from falling in the event of delamination. A third potential problem is that epoxy can weaken under excessive heat or fire and the epoxy set threaded inserts which support the attachment clips could become ineffective.
There have been recent and significant technological developments in the manufacture of thin stone panels which result in slabs with a thickness of only 5/16″ (7 mm+) or ⅜″ (9 mm+) which are reinforced with nettings of fiberglass or expanded steel mesh bonded to one face of the stone slab with epoxy in a vacuum or impregnation process. These thin reinforced slabs are produced in the full block sized dimensions up to about 5 ft. by 10 ft. which is a limitation imposed by the common practice in the stone quarrying industry of extracting and cutting blocks of raw stone into cubic shapes measuring approximately 5′ by 5′ by 10′. These cubic shapes fit into the stone gangsaws which are standard in the industry and which transform the cubic blocks into slabs. When they are polished the thin reinforced stone panels present the outward appearance identical to the much thicker slabs 1¼″ and 2″ thick as used in conventional construction. At present these thin panels are produced by two different Italian manufacturers using different manufacturing processes and may be referenced by U.S. Pat. No. 5,670,007 issued to Marcello Toncelli, inventor, on Sep. 23, 1997 and entitled “Process For The Production Of Reinforced Slabs Of Stone Materials” and by U.S. Pat. No. 5,131,378 issued to Giuseppe Marocco, inventor, and assigned to Tecnomaiera S.r.l., Italy, on Jul. 21, 1992 and entitled “Method For The Production Of Reinforced Panels From A Block Of Building Material, Such As Stone”.
These thin reinforced panels of stone, either marble, granite, or limestone, can be used directly in small dimensions on interior surfaces as flooring tiles or wall paneling applied with various types of adhesives as in conventional construction. The mechanical properties of the thin reinforced panels are generally superior to those of unreinforced thicker stones as used in conventional construction. The reinforcing process transforms the thin sheet of brittle stone into a strong, lightweight, non-brittle (ductile) and impermeable panel which is well suited for use as exterior building cladding. But while the thin reinforced stone has found a widespread market for inteior use as floor tiles and wall paneling, it has not seen the same success in the field of exterior wall cladding. In order to find a wider market and to be successfully utilized on exterior walls, the thin stone must be incorporated into a wall system which is compatible with today's construction methods. The present invention addresses and solves this problem.
For exterior cladding there are obvious advantages in the use of thin reinforced stone panels only ⅜″ thick, weighing only 5.5 psf, instead of the much heavier conventional unreinforced stone 1¼″ or 2″ or even 4″ thick weighing from 16 to 52 psf. Among these advantages are the reduction of jobsite labor and general construction time and overhead because of the ease of handling due to the lightness of weight, and the savings in construction due to less weight being imposed on the building structure. The challenge is to adapt the thin lightweight reinforced stone panels to the methods of building construction, particularly exterior wall cladding, which are in use today in the industry and to make them structurally resistant and accommodative to the external forces of wind loading and movements due to temperature variations and the seismic forces which they could be subjected to when used on the facade of multi-story buildings. The present invention addresses these challenges and provides greater utility and the opportunity for a far wider usage of the thin reinforced stone panels on the construction market.
The present applicant and inventor of the current invention, has previously invented a simple framing system to enable the thin reinforced stone panels to be utilized in curtain wall construction and this was commercialized under the trade name “RS300 Wall Cladding System”. This system was developed several years ago while applicant was employed at Marble Technics Ltd., a USA division of an Italian company, Tecnomaiera S.r.l., one of the developers of the thin reinforced slabs previously referred to above re U.S. Pat. No. 5,131,378 issued to Giuseppe Marocco. Marble Technics ceased operations in 1996 The present invention is an improvement over the prior RS300 system, which was never patented, and addresses a much wider range of possible uses in the art of building construction. It is a much more highly developed wall system.
The RS300 Wall System consisted primarily of a basic extruded aluminum shape which performed as a perimeter frame for the panel as well an intermediate structural stiffener. The frames are adhered to the back face of the stone panel by means of high performance structural silicone. The perimeter frame, while providing structural reinforcement, also provides protection for the thin vulnerable edges and corners of the stone panel as well as a means of attachment to the building structure by use of mating clips which are nested into the frame shape and are connected in turn to the building structure or the curtain wall frames by mechanical means such as screws. After a limited amount of actual use in the field it became obvious that the RS300 System, in its basic simple format, had serious shortcomings. The system was conceived for use primarily in simple flat panel curtain wall facades and to be incorporated into existing aluminum curtain wall systems. The 1″ total thickness of the stone panel and frame together were intended to match and be interchangeable with the 1″ thickness of typical double glazing panels commonly used in most curtain wall facades so that both stone and glass could be used in the same glazing frame. It is now realized that the architectural design requirements of today's buildings, particularly the smaller low-rise suburban office buildings, are much more diverse than the simple flat panel facades. This is especially true when the problem is to renovate by recladding or overcladding an existing facade without necessarily removing the existing facade. The light weight of this thin stone cladding system very often makes such an approach structurally feasible and economically desirable.
Architects are designing more complex profiles into their building exteriors in the cornices, parapets, copings, sills, returns, column covers, etc. In conventional construction these more complex profiles are achieved with traditional stone using 1¼″ and 2″ thick slabs and sometimes with even more massive pieces by employing various metal clips in the edges of the thicker stone attached to back-up support frames usually of structural steel and sometimes using epoxy adhesives to cement stone pieces together to achieve the desired results. The basic RS300 system does not have the capability to reproduce the many features and profiles required to solve the various design problems. To reproduce the wide variety of profiles found in architectural designs the thin ⅜″ reinforced stone requires a specially designed system with adaptability and flexibility to achieve desired results and produce the same visual effect as the thicker traditional stone and this is the objective of the present invention which is an improvement over the RS300 system and which takes into consideration the problems of the current architectural designs which the prior art system is unable to do.
Other shortcomings of the RS300 system were structural in nature. As previously discussed, the basic aluminum perimeter frame was designed to be slightly more than ⅝″ thick in order to combine with the thickness of the stone panel to reach a combined total 1″ thickness in order to match the 1″ thickness of the double glazing panels. However, this was an objective that turned out to have little value because that particular requirement was most infrequent. The finished panel could pass required structural tests but the allowed bending under pressure was greater than desirable which was a factor of the bending strength of the ⅝″ thick perimeter frame of the RS300 system. Another weakness occurred at the corner intersection of the perimeter frames. The interlock clip, which was designed to provide a structural connection between the two perimeter frames at the corner intersection or between a perimeter frame and a stiffener, allowed excessive movement away from the plane of the panel which could produce a bending along the inside line of a perimeter frame at the intersection. This was a defect in its design which could cause fracture in the stone when the panel was subjected to bending pressure due to the live loads or stresses during handling, lifting, packing, and transportation. Another shortcoming occurred with the two-piece panel clamp which was designed to provide a positive mechanical connection between the stone panel and the aluminum frames. This panel clamp turned out to be excessively complicated and difficult to properly install and therefore proved to be ineffective.
In summary, the original RS300 system did not contain sufficient flexibility and scope to solve the many building facade problems which can be encountered in actual practice and moreover it had some structural weaknesses which need to be addressed. The present invention is an improvement over the prior RS300 system and an extension of its capabilities while maintaining its basic concept.